Hello! I have created a PHP file that acts as a Sitemap. It uses a Flarum website's API to gather information about tags, discussions, and users. By default, it will try to use cURL to query the API, and fall back to file_get_contents.
The Git repository containing the file as well as directions is available here. A direct link to download the ZIP is here.
Basically, all a person needs to do to install it is change the URL variable in the PHP file to their own Flarum website URL, optionally configure the other settings, upload the file to their webserver, and link to it.